Cell — Structure and Functions

What you'll learn

• What a cell is; who discovered it.
• The main parts of a cell and their functions.
• Plant cell vs animal cell — differences.
• How cells are organised into tissues, organs, organ systems.

Key concepts

The cell — basics

• Cell: smallest living unit capable of carrying out all life processes.
• Discovered by Robert Hooke (1665) — observed cork cells; coined the word "cell."
• Cell theory (Schleiden, Schwann, Virchow):
  1. All living things are made of cells.
  2. Cell is the basic unit of life.
  3. All cells arise from pre-existing cells.
• Unicellular organisms: one cell does everything (Amoeba, Paramecium, bacteria, yeast).
• Multicellular organisms: many specialised cells (humans have ~37 trillion cells).

Parts of a cell

Cell membrane (Plasma membrane)

• Present in all cells (plant and animal).
• Thin, flexible, semi-permeable (selectively permeable) barrier.
• Controls what enters and exits the cell.
• Made of lipids and proteins.

Cell wall (plants only)

• Outside the cell membrane in plant cells.
• Made of cellulose — rigid, tough.
• Provides structural support and protection.
• Fully permeable (lets everything through; plasma membrane does the selective job).
• Absent in animal cells.

Cytoplasm

• Jelly-like fluid filling the cell between membrane and nucleus.
• Contains water, salts, dissolved nutrients, enzymes.
• All organelles are suspended in the cytoplasm.
• Site of many chemical reactions.

Nucleus

• Control centre of the cell — directs all activities.
• Bounded by nuclear membrane (double layer with pores).
• Contains chromosomes (made of DNA) — carries genetic information (hereditary instructions).
• Nucleolus: dense region inside nucleus; makes ribosomes.
• Most cells have one nucleus; mature red blood cells have none; some muscle cells have many.

Mitochondria

• Rod-shaped organelles; "powerhouse of the cell".
• Release energy from food through cellular respiration:

  Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP)

• Have their own DNA — evidence they were once free-living bacteria (endosymbiosis).
• More mitochondria in cells that need more energy: muscle cells, liver cells.

Chloroplasts (plant cells only)

• Green organelles containing chlorophyll (green pigment).
• Site of photosynthesis.
• Like mitochondria, have their own DNA.
• Give plants their green colour.

Vacuoles

Endoplasmic Reticulum (ER)

• Network of membranes connecting nucleus to cell membrane.
• Rough ER: has ribosomes attached; makes proteins.
• Smooth ER: no ribosomes; makes lipids, detoxifies chemicals.

Ribosomes

• Tiny granules; no membrane.
• Site of protein synthesis.
• Found in all cells (prokaryotic too).

Golgi Apparatus

• Stack of flattened membrane sacs.
• Packages and ships proteins made by rough ER → to where they are needed.
• Makes lysosomes.

Lysosomes

• Contain digestive enzymes.
• Break down worn-out organelles, bacteria, food particles.
• "Suicide bags" — if ruptured, digest the whole cell (autolysis) — occurs during cell death.
• Mainly in animal cells.

Plant cell vs Animal cell

Organisation of cells

Cells do not work alone in multicellular organisms. They are organised:

Cell → Tissue → Organ → Organ System → Organism

Cell division

• Cells reproduce by division to:
  • Allow growth.
  • Replace old/dead cells.
  • Repair damaged tissue.
• Mitosis: produces 2 identical daughter cells (same chromosome number) — growth and repair.
• Meiosis: produces 4 daughter cells with half chromosomes — makes sex cells (sperm, egg).

Quick check

• Who discovered cells? What did he observe?
• What is the function of the nucleus?
• Name three differences between plant and animal cells.
• Why is the mitochondrion called the powerhouse of the cell?
• Arrange in order from smallest to largest: organ, cell, organ system, tissue, organism.

Open the Practice tab for graded questions on Cell Structure.